The present invention relates to coherent imaging systems using vibratory energy, such as ultrasonic or electromagnetic waves, and, more particularly, to novel methods for facilitated correction of errors in data conversion, data transfer and the like functions in a vibratory energy imaging system.
Methods and apparatus for fully digital beam formation in phase-array coherent imaging systems are now well known; one such system, for use in ultrasonic medical imaging and the like, is described and claimed in U.S. Pat. No. 4,839,652, issued June 13, 1989, assigned to the assignee of the present application and incorporated herein in its entirety by reference. Such an imaging system utilizes a phased array sector scanner (PASS) to rapidly and accurately sweep a formed beam of vibratory energy. The desired beam pointing accuracies are obtained by maintaining an accurate set of phase relationships, which are, in fact, a set of time delays between the various ones of a plurality N of transducer elements of the PASS array. By decoupling the required phase accuracy and time delay accuracy from one another, the signals can be coherently summed with greater accuracy. However, proper beam formation requires that both the necessary time resolution and amplitude resolution be provided in each channel, so that the at least one analog-to-digital converter (ADC, utilized for converting the analog RF signal from each channel transducer, at any instant, into a digital data word for processing) in each channel carry out conversions at a sample frequency of at least two times, and usually four times, the maximum operating imager frequency. In an ultrasonic medical imager utilizing signals of up to 10 MHz, each of the N channels (where N is presently on the order of 64) requires the use of at least one ADC of 7 or 8 bit output resolution, and operates at a 20 or 40 MHz sampling rate; those skilled in the art will immediately utilize that seven or eight bit ADC resolution is insufficient to provide the at least 70 dB of instantaneous dynamic range required in each channel of the imaging system. A method and apparatus for utilizing a linear ADC of lesser resolution for realizing a large imaging system dynamic range is described and claimed in co-pending application Ser. No. 207,532, filed June 16, 1988, now U.S. Pat. No. 5,005,419 assigned to the assignee of the present invention and also incorporated here in its entirety by reference. In the invention of that application, predetermined nonlinearity is provided in front of the linear ADC, to compress the analog signal prior to conversion to a digital data word; the digital data word is then further processed in accordance with another non-linear mathematical function which is selected to be the inverse (expansion) of the previously-employed mathematical (compressive) function, so that the value of the expanded digital data words are again linearly related to the value of the input analog RF (echo) signal voltages provided to the input of the compressive amplifier. The inverse (expansion) non-linear relationship may be provided in the ADC itself, or may be provided in a subsequent stage, which may utilize a look-up table approach. The latter approach is particularly desirable if a compression amplifier approximates a power law function and a static-random-access-memory (SRAM) circuit provides an output signal which preserves both the input sign and the inverse power law exponent. For example, utilizing an ADC with a 7-bit-wide output data word, in a system requiring an 11-bit-wide data word for realization of the required instantaneous dynamic range, the power law (i.e. log(V.sub.out)=k log (V.sub.in)) results in k=0.6 for the compressive stage, and an expansion stage constant k'=1/k=5/3.
While the method for operation of the abovedescribed subsystem (i.e. compression analog amplifier, ADC and expansion, digital random access memory (RAM) look-up remap stage) allows the instantaneous system dynamic range to be increased to the desired level, utilizing the preselected nonlinear compressive/expansive complementary functions, we have found that other methods can be utilized with a wide range of channel apparatus to allow the same inversion function to be provided simultaneously with the ability to remove many other system nonlinearities, and to also both remove certain classes of nonlinearities generated by imperfections in the apparatus of each of the plurality N of channels in the system and correct for channel-to-channel differences. Thus, it is highly desirable to provide methods for achieving simultaneous multiple functions, including inverse dynamic range decompression correction of ADC and other channel nonlinearities, and for correcting channel-to-channel gain differences and the like functions in each of a plurality of channels of an imaging system.